Insulating circuits that have large noise immunity and are capable of transmitting analog signals with high integrity are necessary in semiconductor devices used to control power devices. Although semiconductor devices including optically-coupled insulating circuits such as photocouplers and the like that optically transmit signals have excellent noise immunity because the output is completely electrically insulated from the input, such semiconductor devices are not suited to the transmission of analog signals. For example, although IC output-type photocouplers that include a signal processing circuit on the receiving side are used widely to transmit digital signals, analog signals cannot be transmitted with high integrity. Also, insulating circuits such as transistor couplers, etc., that are used to transmit analog signals unfortunately have distortion of the signal waveform caused by nonlinearity and are not suited to transmitting analog signals with high integrity.
Therefore, technology has been developed to add an analog/digital (AD) conversion circuit to the transmitting side of the optically-coupled insulating circuit to transmit the analog signal with high integrity. In other words, the effect of the nonlinearity of the optical coupling unit can be avoided when transmitting by converting the analog input into a digital signal; and the analog transmission is possible with high integrity.
In the case where a delta-sigma conversion circuit is used as the analog/digital conversion circuit, it is desirable to increase the clock frequency of the AD conversion to transmit the information included in the analog signal with high integrity. To have one optical signal path in the optically-coupled insulating circuit, a modulation circuit to superimpose the sampling clock and the digital data into a signal of one series is used in the analog/digital conversion circuit. When the optical transmission is performed using pulse width modulation as the modulation method, there are cases where demodulation errors occur on the receiving side due to pulse width distortion as the sampling clock frequency approaches the response limit of the optical coupling unit. Therefore, an optically-coupled insulating circuit capable of suppressing the demodulation errors by ensuring the transmission margin of the optical coupling unit and realizing signal transmission with high integrity and a semiconductor device including such an optically-coupled insulating circuit are necessary.